This invention relates to a high frequency heating apparatus such as a microwave oven wherein a commercial power is converted by a frequency conversion section to a high frequency power, which power is supplied to a magnetron through a step-up transformer to drive the same, and more particularly to such a high frequency heating apparatus wherein the magnetron can be normally driven in spite of different commercial power supplies.
Generally, a microwave oven is known as a high frequency heating apparatus wherein a commercial high frequency power is converted by a frequency conversion section to a high frequency power, which power is supplied to a magnetron through a step-up transformer to drive the same, thereby high frequency heating a food to cook the same. In the conventional microwave oven, the magnitude of a high frequency power depends upon the magnitude of an anode current of the magnetron and the magnetron anode current depends upon an "on" period of a frequency converting switching element of the frequency conversion section. In most microwave ovens, therefore, the "on" period of the switching element is determined in accordance with the voltage of a single commercial power supply so that the magnetron anode current is maintained at a preselected value. For the purpose of switching heating modes, the magnetron is driven continuously or intermittently without controlling the magnitude of the magnetron anode current.
When the conventional microwave oven as described above is used in a region where the commercial power supply voltage is different from that determined for the microwave oven, the magnetron cannot be normally operated when an input voltage is too low or an extremely large current flows into the magnetron when the input voltage is too high. Consequently, the consumers of the microwave ovens need to have the microwave ovens repaired or to buy new ones when they move from a region to another. Furthermore, when a house is furnished with both 100 V and 200 V power supplies, an error in connection causes the microwave ovens to burn out.